This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Structural study of bacteriophage N4 RNA polymerase transcription initiation complex We are using bacteriophage N4 RNA polymerase (N4 RNAP) as a model system to understand transcription mechanism. We have solved apo enzyme structure at 2.3A resolution by using SeMet-protein MAD method and dataset obtained at X25/NSLS. Recently, we have solved N4 RNAP-DNA complex structure by molecular replacement. The dataset was collected with home source X-ray and the resolution is 2.4A. Nobody solved the structure of RNAP-DNA-substrate complex at transcription initiation stage (transcription initiation complex). We prepared initiation complex crystal by co-crystallization and found it diffracts at 2.4A resolution with home source X-ray. To understand molecular detail of transcription imitation reaction, we want to request beamline time of F1/CHESS to collect transcription initiation complex crystal dataset at atomic resolution. The structure will be the first snap shot of transcription initiation stage and it tells us why RNAP is able to carry out de novo reaction. This project is funded by NIH.